One system of process control known in the prior art and now in general use is disclosed in U.S. Pat. No. 2,949,273, issued Aug. 16, 1960 to C. G. Roper, et al. These presently-available devices are designed to use final controlling means of the "spring loaded" type, in which the force of a spring tending to decrease the process variable (e.g., close a valve) is balanced by a force or "demand" proportional to a variable output or signal from the controller, which force tends to increase the process variable (e.g., open a valve). The output signal from the controller is ramped up or down by negative or positive deviations of the process variable as compared to the set point of the process, and the controller output signal remains constant if there is no deviation of the process variable from the set point.
Thus, in present control devices, the position of the controlling means (e.g., openness of a valve) is proportional to the magnitude of the output signal from the controller. Certain refinements to available control circuitry, known as "rate", "reset" and "proportional band" have added to the sensitivity and responsiveness of the presently available systems, but these improvements have not eliminated the shortcomings and operational problems of the current systems in certain applications.
For example, the presently available spring-loaded control devices have not been successful in controlling largesized control means (such as large valves), which cannot be spring-loaded but rather must be power-operated in both directions by electric, hydraulic or pneumatic means. To adapt the presently available control devices to such power-operated control means, a position-sensing circuit is employed to measure and compare the output or demand signal from the controller to the position of the control means (e.g., the openess of the valve) as measured by a position-measuring device, and to cause the control means to operate when their difference exceeds a predetermined tolerance or "dead band". The demand from the controller must move out of the deadband of the positioning circuitry before any signal is developed to operate the control means (e.g., open or close a large valve).
Further compounding such problems is the comparatively slow operating speed of power-operated control devices, such as electric gear motors, which results in such time lag from receipt of deviation signal to actual physical control activity that the control means does not begin to control the process variable soon eough after a deviation is detected, so that the deviation remains unchecked for a period of time. Because the deviation of the process variable has not been corrected, the deviation amplifier sends stronger and stronger position demand signals (called "ramping up" the demand signal) to the control means, often stronger than desired or required for effective control.
This stronger than desired control signal caused by the ramping up of the demand signal often causes the control means to apply a corrective force of longer duration than desired to the controlled variable, resulting in an over-correction of the process variable.
As the value of the (over-corrected) process variable passes through zero deviation and actually becomes an opposite deviation, an opposite control force should actually be called for. However, because the demand signal has been ramped up too far, the control means continues to apply the original controlling force (e.g., continues to call for opening the valve) when the opposite control force (i.e., close the valve) is actually required. This process of constant ramping up and over-correcting results in a process control anomaly known in the trade as "hunting". Hunting can be reduced somewhat by decreasing the sensitivity of the control system, but over-all control accuracy is sacrificed.
Another undesirable process control anomaly inherent in presently available process control systems is a phenomenon termed "bumping". Bumping occurs when a sudden and significant change in the value of the process variable occurs, such as a large change in flow rate, or the process set point is changed, such as changing from automatic control to manual without having changed the manual set point to approximate the expected new process set point value, or starting the process in the automatic control mode without first manually adjusting the process up to approximately the desired set point. These kind of significant changes in the process control set point or the process variable result in shock to the control device and severe hunting, referred to in the trade as bumping.
The present invention operates independently of the position of the control means and therefore there is no need for any positioner circuitry, even on the largest power-operated control devices. Thus, a key advantage of the present invention is that it accurately and uniformly controls a given process variable with virtual elimination of hunting, and sudden and significant changes in the process variable, and sudden changes in the process set point can be accomplished without bumping. The response of the control means is rapid and continuous where there is a large process deviation, and gradual, intermittent, proportional and accumulative where there are smaller process deviations. The process is controlled much more uniformly than heretofore possible, and the process settings may be manually or automatically "dialed" to higher or lower settings without losing control. In addition, there is no requirement for the complicated rate, reset or proportional band circuitry that is required to render the presently-available process control devices to an acceptable level of sensitivity. Further, the difficulty attendant to placing these difficult to understand and difficult to adjust circuits in the hands of plant operating personnel is eliminated with the present invention.